Generally, Novolak-type phenol resins are prepared by polycondensation of a phenol and an aldehyde in the presence of an acid catalyst.
Novolak type phenol resins obtained by this method generally have a number average molecular weight of 250 to 800, at most 1000, and have a low melting point. Therefore, when these novolak type phenol resins are hardened singly in the presence of a hardener or are hardened together with other kinds of hardening resins and optionally together with a filler and other additives, any hardened resin products having good heat-resistance and good mechanical characteristics cannot be obtained. Other novolak type substituted phenol resins obtained by polycondensing a substituted phenol having an alkyl group or a halogen atom in its p- or o-position with an aldehyde in the presence of an acid catalyst, have a chemical structure similar to the above-mentioned novolak type phenol resins, and have a number average molecular weight of 250 to 800, at most 1200, and a low melting point. Analogously to the above-mentioned novolak type phenol resins, when these novolak type substituted phenol resins are hardened together with other kinds of hardening resins and optionally with a filler and other additives, no hardened resin products having good heat resistance and mechanical characteristics can be obtained either.
As mentioned above, novolak type phenol/formaldehyde resins prepared in the conventional manner generally have, a low number average molecular weight of 250 to 800, at most 1200 or less. It is reported, however, that when these novolak type phenol/formaldehyde resins having a low number average molecular weight were fractionated, slight amount of novolak type phenol/formaldehyde resins having a higher molecular weight of about 3,000 to 10,000 or so was found to be contained therein. (cf. "Lecture on Plastic Materials", (15), `Phenol Resins`, pp. 14-24 (by Shinichi Murayama), Nikkan Kogyo Newspaper Press Co of Japan; J. J. Gardikes, F. M. Konrad, Am. Chem. Soc. Div., Org. Coating and Plastics Chemistry 26 No. 1 131-137 (1966)). However, these high molecular novolak type phenol/formaldehyde resins thus obtained by such fractionation have a narrow molecular weight distribution, and in addition, partially crosslinked and gelled products tend to be formed in the resins as a contaminant, since the phenol components constituting the resins are trifunctional. Therefore, even if these high molecular novolak type phenol/formaldehyde resins are blended with epoxy resins to obtain resin compositions, which are then hardened, it is impossible to obtain hardened products having sufficiently improved and high heat resistance and mechanical characteristics.
The number average molecular weight of novolak type alkylphenol resins obtained by polycondensing a bifunctional alkylphenol, such as an o-alkylphenol or a p-alkylphenol, with an aldehyde in the presence of an acid catalyst falls, in general, within the range of 250 to 800, and is at most 1200, as mentioned above. Some attempts have heretofore been made in order to obtain high molecular weight novolak type substituted phenol resins, but in every case, the resulting novolak type alkylphenol resins have a number average molecular weight of at most 1200, and thus, no novolak type alkylphenol resin having a sufficiently high number average molecular weight has been obtained yet (cf. F. S. Granger, Industrial and Engineering Chemistry, 29 860-866 (1937); J. B. Nierderl and I. W. Ruderman, Journal of American Chemical Society, 67, 1176-1177 (1945); R. F. Hunter and V. Vand, Journal of Applied Chemistry (London), 1, 298 (1951), etc.). Novolak type alkylphenol resins described in the above-mentioned publications have a low number average molecular weight and a low melting point although they have a chain-like or linear molecular structure; and therefore, even if these resins are blended with epoxy resins to obtain resin compositions, it is impossible to improve the heat resistance and the mechanical properties of hardened products of the compositions because of the same reason as mentioned above.
One of the reasons why a phenol cannot be highly polymerized to form a high molecular weight polymer is that the condensation of a phenol with an aldehyde may occur at two ortho positions and one para position (three positions in total) to the phenolic hydroxyl group of the phenol, resulting in that gelation of polycondensed products tends to occur with ease.
Under the circumstances, some other attempts have been made in order to obtain high molecular weight resins, where a monosubstituted phenol, which has a substituent at the ortho or para position to the phenolic hydroxyl group, is used as one of the starting materials so that there can be only two reactive sites in the used phenol and the occurrence of gelation can be prevented. For example, it is reported in Journal of Polymer Science, 20, 75-88 (1956) by W. J. Burke and S. H. Ruteman, et al. that high molecular weight novolak-type chlorophenol resins having a number average molecular weight of 1600 or more, or of 3300 or more, were obtained by polycondensing parachlorophenol with formaldehyde; and it is also reported in Journal cf Polymer Science, 32, 221-228 (1958) by W. J. Burke and S. H. Ruteman that the polycondensation of parachlorophenol with formaldehyde results in the formation of high molecular weight novolak type chlorophenol resins, the acetylated products of which have a number average molecular weight of 1610 or more, or of 3640 or more. However, these high molecular weight novolak-type chlorophenol resins were afterwards denied by other scholars who studied these resins, and the acetylated products have then proved to be low molecular weight novolak type chlorophenol resins having a number average molecular weight of 1250 or less ("Minoru Imoto and Keikichi Uno, Lecture on Theory of Polymerization Reaction, (8), `Addition-Polymeriztion and Addition Condensation`, Kagaku Dojin; Minoru Imoto and Sjinichi Nakade, Bulletin Chemical Society of Japan, 36, 580-585 (1963)" are referred to.).
On the other hand, it is reported in Kogyo Kagaku Zasshi (Magazine of Industrial Chemistry), 66, 95-99 (1963) (by Hideo Narazaki) that novolak type resins having a number average molecular weight of 1,555, 2,735 and 4,560 were obtained by polycondensation of nonylphenol and paraformaldehyde in benzene or toluene in the presence of p-toluenesulfonic acid as a catalyst. However, the formation of the resin having a molecular weight of 4,560 is noted to be accompanied by the formation of solvent hardly soluble components (or gels), and in addition, the report mentions that when p-cresol and nonylphenol were blended in a molar proportion of 2:1 and analogously polymerized, only a resin having a number average molecular weight of 1,355 was obtained.
These reports show that high molecular weight resins may be obtained from nonylphenol used as a starting material of the substituted phenol, which has a high molecular weight, whereas only relatively low molecular weight resins are obtained from cresol, which has a low molecular weight, even though the polymerization degree is the same in both the cases, and that, in the latter case using cresol, if the synthesis of higher molecular weight resins from cresol is intended the formation of solvent insoluble components is inevitable. This, in this manner, it is impossible to obtain any high molecular resins from the cresol
Japanese Patent OPI No 113/82 discloses a process for the manufacture of high molecular weight orthocresol-novolak resins, and it describes that a linear and high molecular weight orthocresol-novolak resin which has a number average molecular weight (Mn) of 2,010 and which is soluble in N,N'-dimethylacetamide is obtained by reaction of an orthocresol-novolak resin having a number average molecular weight (Mn) of 550 and 37%-formalin as a bifunctional chain extender, in the presence of nitric acid and in toluene as a solvent at 175.degree. C. under pressure, and that the softening point of the obtained resin is 134.degree. C.
However, the softening point of said resin is low or 134.degree. C., and when this resin is blended with an epoxy resin and is hardened, the thermal deformation temperature of the hardened product also is low. In addition, when a paracresol-novolak, paratert-butylphenol-novolak or parachlorophenol-novolak resin is used as a starting material novolak resin, instead of the above used orthocresol-novolak resin, and is polymerized analogously, only novolak-resins having Mn of at most 2,000 to 2,120 may be obtained in every case, and it is difficult to obtain higher molecular weight resins.
The above-mentioned Japanese Patent Application (OPI) exemplified the use of o-dichlorobenzene, diphenyl ether and decalin, in addition to toluene, as a solvent. However, o-dichlorobenzene has a higher boiling point and is toxic, and therefore is difficult to handle; and diphenyl ether is solid at normal temperature and thus is also difficult to handle. Decalin is disadvantageous in that the solubility thereof in high molecular weight cresol-novolak resins is low. It is also reported in the Japanese Patent Application (OPI) that the high molecular weight orthocresol-novolak resins obtained by the method described therein precipitate out when their toluene solution is put into methanol, and thus the resins are noted to be insoluble in methanol.
Japanese Patent Application (OPI) No. 92908/81 discloses high molecular weight orthocresol/paracresol copolymerized novolak resins and a method for preparation of such resins. This mentions that a linear, high molecular weight orthocresol/paracresol copolymerized resin which has an Mn of 1,930 and which is soluble in dimethylacetamide is obtained by reacting an orthocresol-novolak resin having an Mn of 550 with 2,6-dimethylol-p-cresol in the presence of nitric acid and in o-dichlorobenzene as a solvent, at 175.degree. C.; and that the softening point of the obtained resin is 134.degree. C.
However, resins which can be obtained according to the method described in the above publication are only orthocresol/paracresol block copolymers containing orthocresol block copolymer components. Accordingly, the softening point of the copolymer resin obtained by the prior art method, which has an Mn of 1,930 and has a molar ratio of orthocresol to paracresol of 8/2, is 134.degree. C. Japanese Patent Application (OPI) No. 92908/81 further described that the starting materials are reacted in o-dichlorobenzene and the reaction mixture is then put into methanol to remove the remaining nonreacted substances, and thus the formed resin is noted to be insoluble in methanol. Whereas, the random copolymerized novolak resin of the present invention which has the same molecular weight and the same molar ratio of orthocresol/paracresol (=8/2) as the resin of the above-described publication is soluble in methanol.